Anaerobic adhesive compositions are well known for their ability to remain in a liquid, unpolymerized state in the presence of oxygen and to cure to a solid state upon the exclusion of oxygen. Early work on anaerobic adhesive compositions concentrated on developing a cure system which improved the speed and/or bond strength of the adhesive composition. Various cure systems for anaerobic adhesive compositions have been developed to primarily focus on efficiently performing the redox reaction, which is the basis for anaerobic chemistry. Hydroperoxides were found to serve as a catalyst for the generation of a free radical. For example, U.S. Pat. No. 2,895,950 to Kriebel, discloses the inclusion of hydroperoxides in amounts of 0.1 to 10% by weight in anaerobic adhesive compositions to achieve faster cure times. Amines, used in amounts up to about 10% by weight, are also disclosed in the '950 patent as accelerators to generate free radicals of the peroxide.
Various accelerator compounds, such as tertiary amines, imides, polyamines, cyclicamines and arylamines, have been successfully included in anaerobic adhesive compositions to further increase cure speeds and completion of cure. See, e.g., U.S. Pat. No. 3,041,322 (tertiaryamines), U.S. Pat. No. 3,046,282 (imides), U.S. Pat. No. 3,203,941 (polyamines), and U.S. Pat. No. 3,218,305 (cyclic and arylamines), all to Kriebel. These accelerators were generally used in amounts of about 0.05 up to about 20%, depending on the type of amine and amount of peroxide present in the anaerobic adhesive composition. Ordinarily, according to Krieble, the amount of accelerator present was significantly less than the peroxide. Additionally, no teaching or suggestion is present in these patents of using specific relative amounts of peroxide to reducing agents.
Those persons of ordinary skill in the art acknowledge that peroxides serve as a free radical generating source which initiate free radical curing of the polymerizable anaerobic adhesive monomer compositions. To increase the speed at which the free radical is generated, accelerators are employed in combination with the peroxides to enhance the speed at which the peroxide free radical is generated. In so doing, the cure speed of the anaerobic adhesive composition is increased.
As anaerobic adhesive compositions have evolved, various combinations of peroxides and accelerators have been used. Recently, such compositions have employed a plurality of compounds as accelerators to serve as reducing agents for the peroxide to increase cure speed. The virtually infinite number of possible combinations of individual components and their relative amounts, have made formulating anaerobic adhesive compositions a complex exercise depending on the properties sought for the composition. While many current anaerobic adhesive compositions exhibit the ability to rapidly polymerize, optimum compositions for a desired set of properties has not been predictable. Thus, given the large number of potential combinations and permutations of anaerobic adhesive composition variables for a given set of desired properties, optimization has neither been practical nor achievable.
U.S. Pat. Nos. 4,287,330 and 4,321,349 to Rich disclose anaerobic adhesive compositions which include a hydrazine accelerator, along with peroxides and amines in amounts previously referred to herein, to increase the cure speed of the anaerobic curing monomer. The hydrazines are disclosed as being useful in amounts of about 0.1-5% by weight. No disclosure is found in the '330 and '349 patents with regard to achieving faster cure speed by adjusting the specific relative amounts of peroxide, hydrozine and amine. Anaerobic adhesive compositions have also conventionally employed primer compositions to speed their cure. On substrates such as stainless steel, zinc, dichromate, and cadmium, which are considered “slow” or relatively inactive materials for anaerobic cure, primer compositions have been considered necessary for quick fixture and cure times required of many applications.
The use of primer compositions requires an additional step prior to applying the anaerobic adhesive composition, which is often inconvenient and costly. Often the solvent used to carry the accelerator component in the primer is environmentally harmful and requires special handling and disposal. Moreover, ordinarily the user must wait until the solvent has evaporated before applying the adhesive.
Designed experimentation (DOE) techniques are known as a method of evaluating multiple factors during scientific experimentation. DOE techniques often use an artificial intelligence component known as a neural network. This technique uses software-generated models to systematically consider the extent to which different variable factors interact by considering many factors simultaneously, while conducting fewer experiments. Commercially available software, such as CAD/CHEM offered by Computer Associates, International, Ilandia, N.Y., produces models to predict the outcome for any given combination of factors. DOE techniques provide a way to combine such factors for products, processes and designing optimized compositions. It is not believed that DOE techniques have been applied to the optimization of anaerobic adhesive compositions. Instead, conventional one-factor-at-a-time (OFAT) techniques have been a common method of learning about the characteristics and improving upon the properties of anaerobic adhesive compositions.
Whereas the Kriebel patents disclosed anaerobic adhesive compositions with relatively few components, current commercially available anaerobic adhesive compositions include many additional components which are believed to participate in the cure mechanism. The ability to determine optimized compositions which are reduced to the least number of components would represent a significant advance in achieving desired properties, as well as representing significant simplification in the manufacturing process, reduction in the material cost and an increase in overall production efficiency. It would be further advantageous if such optimized compositions achieve faster fixture and cure times without the need for independent primer compositions or numerous accelerator combinations. Moreover, there is a need for a method of generating an optimized composition based on a set of desired properties of the adhesive, whether these properties be in the uncured or cured form, or in the cure profile.